To provide for easier entry, there is no "lip" on the back of many dive boats around here. It's just flat deck all the way to the end, and a very short drop to the water from the edge. In order to keep stuff from falling off the back of the boat while underway, they put a large board, something like a 2x12 or 2x24 across the opening. The board gets lifted off so it's wide open for divers once the boat stops. That board is what he was talking about being removed.My bad. Your comment that “When removed it made a sluice for incoming water.” made me think the covers had been removed
On purpose built dive boats, it's an excellent system. On fishing/pleasure craft converted for dive it can be a little less ideal, requiring more skill to operate the boat safely and with reduced capability to handle seas/current... as this incident shows.
Here's an example of how it looks on a purpose-built dive boat:
If you want to know the proper names for the pieces, I'm sure @Wookie could elaborate.